In recent years, as measures against environmental issues, a steep rise in the price of crude oil and the like, the tendency to pursue energy conservation in various industrial products has been accelerated. Accordingly, an improvement in environmental protection characteristic and/or an improvement in energy conservation characteristic, such as achieved by motorization or hybridization, have come to be also required of conventional construction machinery most of which drives a hydraulic pump with the aid of an engine to drive a hydraulic actuator by pressure oil ejected from the hydraulic pump.
Hybridization of the running gear of the construction machine makes it possible to start a generator by use of the engine to charge a battery when the engine produces surplus power, and to use power of the battery to drive an electric motor so that the drive force of the electric motor assists the drive force of the engine when the construction machine is operated at torque exceeding the capacity of the engine. As a result, engine downsizing can be achieved, leading to improved fuel efficiency of the engine, a reduction in emission, a reduction in noise. Further, a regenerative electric power regenerated by the motor can charge the battery during braking, so that effective use of energy can be made. In this respect, energy conservation in the construction machine can be also achieved.
In the related art, a construction machine in which a parallel hybrid drive unit with a parallel connection of an engine and a motor-generator is applied to a hydraulic pump which is a load is proposed (see, for example, Patent Literature 1). In the technique described in Patent Literature 1, the input shaft of the hydraulic pump is connected through a differential to the output shaft of the engine and the output shaft of the motor-generator, and while the engine is operated at most-efficient rated engine speed, the RPM of the hydraulic pump is designed to be freely changed in order to adjust the flow rate. Also, the pump is allowed to be driven by the motor-generator alone without a clutch provided on the output shaft of the engine.
FIG. 7 shows an example of well-known typical wheeled excavators. As seen from FIG. 7, the wheeled excavator of the example includes a machine room 1 rotatably mounted on the top of a chassis 2 which supports front wheels 10 and rear wheels 11 for running. The machine room 1 incorporates an engine 3, a hydraulic pump 4 driven by the engine 3, and an oil tank 25 storing hydraulic fluid used by the hydraulic pump 4. In front of the machine room 1, front members including a boom 12, an arm 13 and a bucket 14 are coupled in series. The machine room 1 is turned relatively to the chassis 2 by supplying pressure oil ejected from the hydraulic pump 4 to a swing hydraulic motor which is not shown. The front members are driven by supplying pressure oil ejected from the hydraulic pump 4 to hydraulic cylinders which are not shown.
A running hydraulic motor 24 and a transmission 6 connected to the hydraulic motor 24 are placed in the chassis 2, and a propeller shaft 7, which extends from the transmission 6 in the fore-and-aft direction of the vehicle body, transfers the power to a front differential 8 and a rear differential 9 coupled respectively to the front wheels 10 and the rear wheels 11. The running hydraulic motor 24 is rotated by pressure oil supplied from the hydraulic pump 4 through a piping joint 38 disposed at the center of rotation of the machine room 1. The torque of the motor 24 is transferred from the transmission 6 through the propeller shaft 7, the front differential 8 and the rear differential 9 to the front wheels 10 and the rear wheels 11, whereby the wheeled excavator 41 runs. The oil pressure with a pressure reduced by driving the running hydraulic motor 24 travels back to the oil tank 25 provided in the machine room 1 through the piping joint 38.